The present invention relates generally to signal amplifiers and applications thereof. The accuracy of many electronic systems is degraded if critical amplifiers cannot supply currents that are sufficient to accurately amplify input signals. For example, signal amplifiers are often required to drive loads which partially or wholly comprise a capacitive load. To generate an accurate amplified version of an input signal across these capacitive loads, an amplifier provides high currents which can accurately alter the output signal's amplitude.
An example amplifier is an amplifier in a multiplying digital-to-analog converter (MDAC) of a pipelined analog-to-digital converter system. Such systems are configured with successive converter stages which each convert an analog input signal to respective digital bits of a final digital code that corresponds to the analog input signal.
Integrated circuit MDACs are often configured with capacitors that are switched in a first portion of each sample period to receive a charge from the residue signal of the preceding converter stage and switched in a second portion of each sample period to transfer this charge to an output capacitor. These charging and transferring processes are generally accomplished with the aid of an MDAC amplifier and the accuracy of these processes is dependent upon the ability of this amplifier to provide high-level currents to the MDAC capacitors.
In general, NMOS devices have higher trans-conductance than PMOS devices for the same current density and relative parasitic capacitance values. As a result, NMOS devices are desirable as the gain devices in an amplifier. Although PMOS devices can also be used, PMOS devices are more often used as passive devices, such as current sources for biasing the NMOS gain devices. In a 65 nmLP process, the PMOS devices have slightly lower intrinsic gain, 6.6 vs. 7.5, as their NMOS counterparts and their trans-conductance is about 50% lower.
In order to increase the intrinsic gain of a stage, the output impedance of the PMOS current source, relative to the NMOS device, is high. An output resistance of 15 kilo-ohms can reduce the NMOS gain by 10%. If a single device were used, then the PMOS intrinsic gain would need to be 51, which would require a prohibitively large gate length and associated device parasitic capacitance. As such, the PMOS current sources can be cascoded. If the desire is to stay within the supply rail of the process, then the output swing would be severely compromised. To overcome this problem, the amplifier's current sources are typically cascoded and operate from supplies greater than the supply rail of the process. However, this causes an increase in power and has the undesirable challenge of making sure all devices stay within their operating breakdown voltage during start-up, shutdown, and overdriven conditions.
Prior designs have also used extended supply voltages to cascode devices, calibration, and miscellaneous gain enhancement techniques. While the latter two are desirable for fine line CMOS MDAC amplifiers designs where the intrinsic gains of the devices are small, using the extended supply voltage increases power and reduces reliability. Accordingly, an amplifier that achieves sufficient open loop gain, while still maintaining closed loop bandwidth, is needed.